Electron discharge device having cavity resonator



Feb. 2, 1954 L. F. BROADWAY ELECTRON DISCHARGE DEVICE HAVING CAVITY RESONATOR v I u A J m H F 4 ILIIHU Patented Feb. 2, 1954 ATENT OFFICE ELECTRON DISCHARGE DEVICE HAVIN CAVITY RESONATOR Leonard Francis Broadw ay, Ickenham, England,

assignor to Electric & Musical Industries Limited, Hayes, England, a company of Great Britain Application January 11, 1946, Serial No. 640,549 In Great Britain January 14, 1941 Section 1, Public Law 690, August 8, 1946 Patent expires January 14, 1961 4 Claims.

This invention relates to electron discharge devices of the velocity-modulation type having cavity resonators.

In this type of electron discharge device an electron beam is caused to pass through a cavity resonator within which electric Waves are set up so as to effect so-called velocity modulation of the beam. The modulated beam may then be passed through a so-called drift space within which the electrons in the velocity-modulated beam become bunched so that the velocity modulation becomes converted to so-called charge-density modulation and thereafter in one form of electron discharge device the thus modulated beam is passed through a further resonator similar to the first-mentioned resonator so as to set up electric waves therein. In another form of device of the above type, instead of employing a further resonator, the beam, after passing through a resonator, can be caused to be reflected back through the resonator, the beam passing through a sufiicient distance to enable bunching of the electrons to occur.

These devices can be employed for amplification purposes, but are specially suitable for use as'generators of oscillations of very high frequencies. In the case of an oscillation generator where two resonators are employed, these resonators can be coupled in any suitable manner so as to cause the production of oscillations, whilst where only a single resonator is employed, the reflected beam in passing back through the resonator likewise serves to cause the production of oscillations. In either case the frequency of the oscillations is determined by the dimensions of the resonator or resonators.

In another form of device of the above type the reflecting electrode which is provided to cause reflection of the electrons is maintained at such a potential that only those electrons below a predetermined velocity are reflected back towards the resonator whilst those electrons exceeding a predetermined velocity are collected and serve to provide an output voltage when the current collected is passed through a suitable impedance. A device of this form is particularly suitable for use as a mixer of high frequency oscillations, the two oscillations to be mixed either being applied to the same resonator or else applied to separate resonators.

In some cases the resonator is in the form of a toroid provided with axial apertures through which a cylindrical-shaped electron beam is passed. It is found that with such a construction the diameter of the apertures has to be small compared with the operating wavelength in order to avoid excessive losses through radiation and due to the transit time of the electrons. For example, if it is desired tQgenerate oscillations of a wavelength of 1d cms. an aperture diameter of the order of 6 mm. is found to be the largest permissible. Consequently, if it is desired to employ the device for the generation of shorter wavelength oscillations, say, of the order of 3 cms., the apertures must be correspondingly reduced and likewise the cross-sectional area of the beam of eiectrons must be similarly reduced and the beam current therefore is very small and the voltage required to produce any appreciable power becomes excessive. This therefore imposes a limit to the power available from known types of devices at frequencies, say, above 6000 megacycles per second.

The object of the present invention is to provide an improved arrangement to enable a larger power output to be obtained at very high frequencies.

According to one feature of the present invention, an electric circuit arrangement is provided including an electron discharge device of the velocity-modulation type comprising a cavity resonator of annular form, a cathode surrounded by or surrounding said resonator and arranged to project electrons in a number of directions radially through a gap in said resonator whereby said electrons are velocity-modulated the gap being either left entirely open or having only a grid or grids joining its opposite edges and lying transversely of the path of the electrons, and a refleeting electrode so arranged and maintained at such a potential as to reflect some or all of said electrons back towards or through said resonator.

An oscillator has been proposed in Figure 15 of United States Patent No. 2,259,690, issued 00- tober 21, 1941, to Hansen et al., having an annular cathode producing two discrete beams of electrons surrounded by a toroidal resonator formed with an annular gap through which electrons from the cathode pass to become velocity-modulated, the resonator being surrounded by a refleeting electrode for reflecting the electrons back of the invention, maybe 'such that the velocitythrough the resonator. In this prior proposal the resonator is provided with a grid at each side of the gap bounded by the internal space of the resonator in such a manner that the field set up at the gap is divided into weak and strong zones, the electrons diverging from the cathode and forming a cross-over at the centre of the gap where a zone of weak oscillatory'field strength exists and then being reflected and diverging to the boundaries of the gap where a zone of strong oscillatory field strength exists due to the particular construction of resonator employed. .The present invention differs from this prior proposal in that no grid is provided to divide the oscillatory field into strong and weak zones. Thus, either the gap is left entirely openaorisprovided only with a grid or grids joining its opposite edges and lying transversely of the ,path of the electrons, this grid or grids merely serving-to prevent spreading of the oscillatory field inknown manner. In the present invention, the ielectronsrpassing from the cathode through the gapzwillbesubiected to substantially the same field as theelectrons which are reflected back through'the'ga'p whereas this is not the case in the above-men- -tioned prior proposal since the electrons passingfrom the-cathode are subjected to the=action of a weak'field whilstthe electrons reflected-back throughthe gap are subjected to the actionof a strong field.

A further resonator alsoof annular form may be provided through whichthe electrons are also caused to pass before being reflectedtowards or through the resonator adjacent said reflecting electrode. l he resonator or resonators employed inthis feature of the invention may be of toroidal form, or alternatively, the resonator or resonators is orare in the'form of a-concentric transmission line or lines.

According to another feature-of the-invention. an electron discharge device of the velocitymodulated type is provided-comprising ahollow resonator or" toroidal form and a-cathode surrounded bysaid resonator and arranged to project astream of'electrons ina number of directions radially through a gap in said resonator whereby said electrons can become velocitymodulated, the gap being left entir'ely'open or being provided only with a grid or grids lying transversely of the path of the electrons and joining its opposite edges.

Preferablyan annulargap is provided in the toroidal resonator 'or resonators and the oathode is arranged to proiectelectrons'throughsubstantially the whole extent of the'gap or gaps, the electron stream-being in'the form of'a sheet.

The disposition of the various electrodes of the device and the potentials which-are applied in operation thereto, "according 'to'either feature modulated beam becomes converted 'into chargedensity modulation, although where the'device is-intended to be employed as amixer of oscillationsit is not necessary for-the velocity-modulated beam to be bunched.

Where a toroidal resonatorisemployed-with a radial flow of electrons it will be appreciated that the current in the --electron beamcan be considerably increased compared with the case in which a cylindrical beam is employed; and also the dimensions ofthe gap at-right angles'to the :path of the electron stream can be reduced with- -out unduly limiting the *electroncurrent.

-In order that the invention -may be clearly understood-and re'adily carried intoefie cait will now be more fully described with reference to the accompanying drawings, in which:

Figure 1 illustrates diagrammatically a vertical section in perspective of one embodiment of an electron discharge device according to one feature of the invention,

Figure 2 illustrates diagrammatically a further construction, in which the resonator is in the form of a concentric transmission line,

Figure 3 illustrates diagrammatically a modification of the arrangement shown in Figure 1, and

Figure 4 is a fragmentary view of a portion of Figure 1 showing a modification.

Referringnow to Figure 1 of the drawing, the device comprisesa'n evacuated envelope l within which thereis arranged an annular cathode 2 mounted between a pair of disc-shaped electrodes 1i whichwhen suitable potentials are applied thereto serve to converge electrons emainating irom the cathode 2 into a sheet-like iorm. Ph'e cavity resonator indicated by the reference numeral 4 is of annular form and is a toroi'd generated by revolving a substantially dumb-bell-shaped figure arranged with its major a-xisparallel'to-the-aXis of*the toroid. A'-gap 5 'is'provided in the reentrant portion oi 'the resonator, as indicated, through which electrons from the cathode proceed in a nuniber or direc- 'tions radially through the 'resonator. "The gap "5 is formed by providingopenings in the opposite walls of the resonator and if desired, a -'g'1'id"01 grids 5a maybe provided 'joining'one-or both-of the pairs of opposite edges of the upper and lower portions of said openings asviewe'd'in-Figure -1. The grid orgrid's 5a will therefore lie transversely of the 'path "of *the electron stream and serves or serve to prevent spreading "of the oscillatory field setup in the resonator. 'The gridslia may beomitted as indicated in Figure 4 to provide an entirely open gap. The-cathode '2 'is-so mounted-that its mediallplane is-co incident with themedial planerpassing"throughthe gap '5. Normally the potential applied inoperation to the resonator '4 'issufficient to accelerate the electrons from the "cathode.

In the-example shown in Figure "1, the electronsafter flowing "through 'gap 5 are reflected back through the gap by the provision of i a refleeting electrode "1 "disposed beyond the gap "and comprising a :pair of ring-shapedelectrodes "which are "maintained 'at a suitable potential negative with respect 'to the cathode 2 and so positioned in relation to the-gap as tocause 'refie'c'tionof the electrons. The:p'otentials required -f or operating the device are shown as *being de-- 'rived conventionally -from a battery Bandpotentiometer' P. The two electrodes I 'may'beconnected together by a surrounding ring, thus forming a single electrode of U-shaped cross- -section. Electrodes of other cross sectional forms may be used, if "desired.

The device shown in "the drawing is intended to beemployed asa generator of oscilla'tions'the frequency of the oscillations being determined by the dimensions 'of the resonator "Ihe electrons when passing through the gap '5 become velocitymodulated in known 'manner, by the "oscillations within the resonator and the -'electrons, after passing throughthe gap, are reflected by the field set up by the "electrode 1 back through the gap 5, the electrons becoming bunched during I their passage and "arrive at the gap 5 in phase with the oscillations already existing in the resonator. -It may I be found that the electrostatic coupling between the upper and lower portions of the resonator 4 is not sufiicient for eificient operation, in which case the two portions may be further coupled by the provision of a concentric line coupling or by some other suitable coupling means. Energy may be derived from the resonator by the provision of a concentric line indicated by the reference numeral 8 and a coupling loop, not shown.

The vertical dimension of the gap 5 as viewed in Fig. 1 is dictated by the frequency at which the device is desired to operate, and it will be appreciated that since the electrons emanate from the cathode in a sheet-like form this distance can be made small whilst at the same time permitting a large electron current to flow, enabling a considerably higher power to be obtained for given voltages. Further, since the electrons spread out over a larger area near the reflecting electrode compared with the previously mentioned kind of discharge device, any space charge effect which tends to destroy the required bunching of the electrons will be reduced.

If desired, a further toroidal resonator 4a similar in shape to and surrounding and concentric with the resonator 4, may be employed, the two resonators in such a case being suitably coupled together by a coupling 8a where the device is intended to be used as a generator of oscillations.

The device shown is not limited in its use as an oscillation generator. For example, the device may be used as a mixer of oscillations, in which case the reflecting electrode 1 may be maintained at such a potential that those electrons below a predetermined velocity are reflected, whilst those electrons exceeding a predetermined velocity are collected by the electrode 1 or by a further target electrode and serve to generate an intermediate frequency output voltage across a suitable impedance. The oscillations to be mixed may be applied to the same resonator or to separate resonators.

Figure 2 of the drawings illustrates a device employing a resonator in the form of a transmission line. In this construction there is arranged within an evacuated envelope 9 a cathode In which is of elongated form, as shown, and from which electrons are emitted in a number of directions radially with respect to the cathode and with respect to the cavity resonator I I. The electrons from the cathode may be focused by the provision of focusing electrodes [2, as shown, which in operation are maintained at appropriate potentials to prevent electrons emanating in undesired directions from the cathode. The resonator II is of annular form and comprises a transmission line having an inner hollow conductor l3 and an outer hollow conductor l4 concentric with the inner conductor, both the inner and outer conductors being provided with annular gaps, as shown, through which electrons from the cathode can pass. The opposed edges of the inner and outer conductors, between which the gaps are formed, are interconnected by grids l5 and I6 formed of wires or rods. The transmission line is short-circuited at its end indicated by the numeral ll, this short-circuit being disposed at a distance from the centre of the grids l5 and it which is equal to an odd number of quarter wavelengths of the frequency of operation. Thus, when a standing wave is set up in the transmission line, the grids are at a potential antinode. Alternatively, this potential antinode can be set up at the grids by employing an open-circuited transmission line, the distance between the opencircuited end and the centre of the grids being equal to a whole number of wavelengths of the frequency of operation. The transmission line H projects through the envelope 9 and is sealed thereto and the projecting end may be provided with a short-circuiting piston [8, by means of which the resonant frequency of the transmission line can be adjusted externally of the envelope. In the example shown in Figure 2 the grids are surrounded by a reflecting electrode H! which is maintained at a potential negative with respect to the potential which is applied in operation to the cathodes") and serves to reflect the electrons back through the transmission line, the operation being similar to the operation of the device of Figure 1. The disposition of the electrodes of. the device shown and the potentials which are applied thereto are such that bunching of the electrons occurs as the electrons are reflected back towards the resonator.

It will be appreciated from Figure 2 that the electrons from the cathode proceed radially through the resonator in a number of directions so that a large beam of electrons can be employed. In this construction of the device the distance between the adjacent edges of the portions of the transmission line through which the gap is formed can be larger than the distance between the adjacent edges of the toroidalshaped resonator shown in Figure 1, owing to the provision of the grids I5 and i5, which confine the high-frequency field, thus permitting a thicker beam of electrons to be employed. The device shown in Figure 2 is intended to be employed as a generator of oscillations, but can also be employed as a mixer of oscillations, by suitably arranging the electrodes and maintaining the reflecting electrode I9 at such a potential that only those electrons exceeding a predetermined velocity are collected to provide an intermediate frequency output. The oscillations to be mixed may be applied to the single resonator H or a further resonator may be employed also in the form of a transmission line surrounding the resonator H and concentric therewith, the oscillations to be mixed being applied individually to the two resonators. Figure 3 illustrates an arrangement in which such a further resonator 4a is provided.

What I claim is:

1. An electric circuit arrangement including an electron discharge device of the velocitymodulation type comprising a cavity resonator of annular form and having openings in opposite walls thereof to form a gap with a zone of constant field intensity, a cathode, said resonator and cathode being concentrically arranged with one of them surrounding the other, means for projecting electrons in a number of directions radially through said zone in the gap whereby said electrons are velocity-modulated, and a reflecting electrode disposed beyond said gap from said cathode and maintained at a potential to reflect at least some of said electrons back towards said resonator, said resonator being in the form of a concentric transmission line.

2. An electric circuit arrangement including an electron discharge device of the velocity-modulation type comprising a cavity resonator of annular form and having a gap formed by openings in opposite walls of said resonator, a cathode, said resonator and cathode being concentrically arranged with one of them surrounding the other, means for projecting electrons in a number of sens-seas directionsmadially through saidgap wherebyisaid electrons :are fiveiocityamodulated, :a .grid joining said opposite edges :of :said ropenings :and lying transversely of :the ipath :of .said electrons, .and a reflecting electrode disposed beyond .said ggap from said cathode :and maintained at ;a poten- 'tial .tozrefiect atileast'somea-of saidelectrons back towards: said resonator.

'3. An electric circuit.arrangementaoeordingto xclaim 2, provided with a further resonator of annular form :disposed rbetween said first-men- .tioned I resonatoreand. saidirefiecting electrode.

4. .An electron disoharge'device of-thevelocity- :modulatinn type nomprising .a cavity resonator 10f toroidal :form and having a gap formed by a -;openings in opposite walls :of said .resonator, ,a cathode, said iresonatorzand cathode being con- ;centrically arrangedwithzoneof them surround- Zing the'0ther,":means for projectingelectrons :ammnber of directions rradiallythroughzsaid gap whereby said electrons are ve1ocity-modulated,-.a grid joining said opposite edgesof said openings gandiying transversely of the .path of said electrons,-and a-reflecting electrode disposed beyond said gap from said cathode.

LEONARD FRANCIS BROADWAY.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 7 2,207,846 Wolff July 16, 1940 2,259,690 Hansen etal Oct. 21, 1941 2,351,744 Chevi ny June:20, 1944 42,372,213 Litton Mar. 27, 1945 2,466,063 Varian Apr. 5, 1949 

